Prostacyclin (prostaglandin I.sub.2, also referred to as PGI.sub.2 for short), a bioactive substance belonging to the arachidonic acid cascade, possesses potent platelet aggregation suppressive action, vasodilating action and other bioactivities. The bioactivities of PGI.sub.2 are antagonistic to thromboxane A.sub.2 (also referred to as TXA.sub.2 for short); its measurement in vivo has been much emphasized. However, PGI.sub.2 is an unstable substance, whose half-life at 37.degree. C. is reportedly about 5 minutes. For this reason, 6-keto-prostaglandin F.sub.1 .alpha. (also referred to as 6-K-PGF.sub.1 .alpha. for short), which is derived from PGI.sub.2 in the absence of enzyme, has been measured in place of PGI.sub.2 (ALAN R. BRASH et al., J. Pharmacol. Exp. Ther., 226, 78 (1983)).
However, because of their low production rates and short half-lives, the concentrations of 6-K-PGF.sub.1 .alpha. in human plasma obtained under resting conditions are in the low picogram range (FitzGerald, G. A. et al., Circulation, 67, 1174-1177 (1983)). Moreover, sampling-induced artifacts and ex vivo synthesis by blood cells make most plasma measurements of primary icosanoids totally unreliable (Carlo Patrono et al., Advances in Prostaglandin, Thromboxane, and Leukotriene Research, Vol. 15, p. 71-73 (1985), edited by O. Hayaishi and S. Yamamoto, Raven Press, New York). Therefore, because the values of 6-K-PGF.sub.1 .alpha. obtained vary widely among institutions, comparison by absolute values has been almost impossible among different institutions [M. Suzuki et al., Japanese Journal of Clinical Medicine, Vol. 43, p. 1082 (1985)].
One method of circumventing such problems is measurement of metabolites that have an extended half-life. Although the 6,15-DK-13,14-DH-PGF.sub.1 .alpha. metabolite of PGI.sub.2 has a longer half-life than 6-K-PGF.sub.1 .alpha., it was expected to precisely reflect changes in PGI.sub.2 concentration [C. Patrono et al., Clinical Research, Vol. 29, p. 276A (1981)].
The object of the present invention is to provide a means for conveniently measuring 6,15-DK-13,14-DH-PGF.sub.1 .alpha., a stable metabolite of prostaglandin I.sub.2 thought of as reflecting the concentration changes therein, with specificity and high sensitivity.
In view of the circumstances described above, the present inventors conducted investigations with the aim of developing a practical means of assaying 6,15-DK-13,14-DH-PGF.sub.1 .alpha., and prepared a monoclonal antibody against 6,15-DK-13,14-DH-PGF.sub.1 .alpha. enabling such assay. The inventors conducted further investigations based on this achievement and developed the present invention.